The invention relates to an apparatus for distributing a liquid across the packed bed of a vapor-liquid contact column.
Packed columns for vapor-liquid contact are used in various operations, such as distillation, fractionation, absorption, stripping and heat exchange. The columns are usually filled with randomly-oriented packing material, but sometimes it is carefully positioned. These operations usually involve counter-current flow of the vapor and liquids in the column, but the flow can also be co-current. Some operations require a column which has only a single bed of packing therein, with one device for distributing liquid onto the packing. In these columns the device is mounted above the packed bed and it is known as a liquid "distributor". Other operations require a column that has more than one packed bed. In this instance the column includes additional distributing devices, which are positioned between the packed sections and are known as liquid "redistributors".
Many conventional liquid distributors are of the trough (or tray) type. These distributors consist of elongate troughs with closed ends, which are spaced apart and arranged in parallel relation. Triangular or rectangular-shaped weirs are cut into the sides of the troughs, or there are spaced orifices along the bottom wall of each trough. Mounted above the troughs are box-like structures referred to as parting boxes or splitters. The distributor assembly described above is mounted in the column above the packed bed. In a vapor-liquid contact operation, the liquid usually enters the parting boxes from a source outside the column, and is fed from the boxes into the troughs. As the liquid overflows the weirs (or passes through the orifices), it falls onto the packing in the column. Vapor entering the bottom of the column rises upwardly through the packing, where it contacts the downflowing liquid and passes between the spaced apart troughs.
The function of the distributors and redistributors in vapor-liquid contact operations is to ensure uniform wetting of the packing material. If the packing is wet unevenly, a condition usually referred to as "maldistribution", the desired transfer of mass and heat between the vapor and liquid phases is, at best, very poor. Maldistribution usually results in poor separation of components in the column, and it can have other detrimental effects, such as material decomposition in processes which utilize temperature-sensitive liquids as one of the reactants.
Maldistribution is a common problem in packed columns having large diameters, that is, wider than about eighteen inches. In addition, this problem is particularly acute in operations, such as vacuum fractionation, where the liquid loading per cross-sectional area of the column is low. In the larger columns the longer span of the distributor troughs makes it much more difficult to hold the troughs in a level position. Most liquid distributing troughs are not precisely level when installed. Later, as the column shifts, the troughs become even further out of level. In a trough which is not level, the liquid collects at the lower end of the trough and the higher end is thus "starved" for liquid.
In vacuum fractionation, and other operations which require a low flow rate of liquid from the trough to the packing, the liquid must drain through very small orifices or weirs in the troughs. The main problem in distributor troughs of this type is that solids in the liquid tend to plug off the drain orifices or weirs.